Large ore grinding mills used in the mining industry operate at low speeds and require drives of several thousand horsepower. Electric motive power systems for the larger sizes of such grinding mills often divide the load between two nominally identical electric motors which drive through pinions at different positions on the periphery of a bull gear coupled to the grinding mill so that the two motors are mechanically connected to the common load by the reduction gearing which reduces the speed to the low value required for the grinding mill. When the two motors are synchronous motors, a sustained unbalance in load between the two motors may exist if the angular positions of the rotors on their respective shafts do not result in the same displacement angle for the two motors. Also, for either synchronous or induction motors, an unbalanced load may oscillate back and forth between the two motors at a frequency corresponding to grinding mill speed and/or multiples thereof as a result of small errors of concentricity or of tooth pitch in the gears or slight misalignment of the shafts. Such inherent inaccuracies cause incremental changes of the displacement angle, or electrical coupling angle of each motor and in the rotational angular velocity of each motor with respect to its revolving stator field, and these incremental changes on one motor are usually out of phase with those of the other motor. Such changes in displacement angle and rotational angular velocity result in pulsating swings of load between the two motors. Such pulsating load swings and the sustained load unbalance cause higher peak loading on the gears and can result in motor overheating, vibration, and damage to the gears.
Twin motor drives for such ore grinding mills are known which utilize synchronous motors because they permit control of power factor and are more economical than induction motors, and certain of such twin synchronous motor mill drives attempt to compensate for undesirable load pulsations between motors by measuring the power inputs to the two motors and continually increasing the field excitation of the motor developing the lower torque and reducing the excitation of the motor developing the higher torque. Such known twin synchronous motor drives require special excitation systems and have high initial cost and high maintenance cost and also increase the peak values of stator current and field current on the motors. Another known motor drive for such a grinding mill utilizes twin synchronous motors with an auxiliary field winding which produces a magnetic field having a polar axis spaced angularly from the polar axis of the main field and attempts to compensate for load pulsations between motors by adjusting the angle of the magnetic axis of the motor field excitation with respect to the rotor poles so as to shift the load torque angle. This requires a very complicated and expensive rotor structure for at least one of the motors and also necessitates an elaborate and expensive control system.